Covalent attachment of the C terminus of ubiquitin to cellular proteins plays a role in a variety of cellular processes. Ubiquitin C-terminal hydrolysis is catalyzed by deubiquitinating (DUB) enzymes and is necessary for several functions, including liberation of monomeric ubiquitin from the precursors encoded by ubiquitin genes and recycling of ubiquitin monomers. There are 2 distinct families of DUBs, ubiquitin-specific proteases (UBPs) and ubiquitin C-terminal hydrolases (UCHs). By screening a human B-cell expression library with the antibodies, the authors isolated cDNAs encoding human UCHL3. The predicted 230-amino acid human UCHL3 protein is 54% identical to UCHL1.In adult tissues, Uchl3 was variably expressed in all tissues examined, with highest expression in testis.